Method and apparatus for using flex circuit technology to create a reference electrode channel

ABSTRACT

A method of creating a sensor that may include applying a first conductive material on a first portion of a substrate to form a reference electrode and depositing a first mask over the substrate, the first mask having an opening that exposes the reference electrode and a second portion of the substrate. The method may also include depositing a second conductive material into the opening in the first mask, the second conductive material being in direct contact with the reference electrode and depositing a second mask over the second conductive material, the second mask having an opening over the second portion of the substrate, the opening exposing a portion of the second conductive material, which forms a working surface to receive a fluid of interest.

CLAIM OF PRIORITY UNDER 35 U.S.C. §119

The present Application for Patent claims priority to ProvisionalApplication No. 60/777,133 filed Feb. 27, 2006, and assigned to theassignee hereof and hereby expressly incorporated by reference herein.

FIELD OF THE INVENTION

The invention relates generally to flex circuit technology. Morespecifically, the invention relates to using flex circuit technology tocreate a reference electrode channel.

BACKGROUND

Flex circuits have been used in the micro-electronics industry for manyyears. In recent years, flex circuits have been used to designmicroelectrodes for in vivo applications. One flex circuit designinvolves a laminate of a conductive foil (e.g., copper) on a flexibledielectric substrate (e.g., polyimide). The flex circuit is formed onthe conductive foil using masking and photolithography techniques. Flexcircuits are desirable due to their low manufacturing cost, ease indesign integration, and flexibility in motion applications.

SUMMARY

The invention relates to a method of creating a sensor that may includeapplying a first conductive material on a first portion of a substrateto form a reference electrode and depositing a first mask over thesubstrate, the first mask having an opening that exposes the referenceelectrode and a second portion of the substrate. The method may alsoinclude depositing a second conductive material into the opening in thefirst mask, the second conductive material being in direct contact withthe reference electrode and depositing a second mask over the secondconductive material, the second mask having an opening over the secondportion of the substrate, the opening exposing a portion of the secondconductive material, which forms a working surface to receive a fluid ofinterest.

The invention relates to a method of creating a sensor that may includeapplying a first conductive material on a first portion of a substrateto form a reference electrode and a second portion of the substrate toform a working electrode, and depositing a first mask on the substrate,the first mask having an opening that exposes the reference electrode,the working electrode, and an area between the reference electrode andthe working electrode. The method may also include depositing a secondconductive material on the reference electrode and in the area betweenthe reference electrode and the working electrode and depositing asecond mask on the second conductive material.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings, wherein:

FIG. 1 is a cross-section view of a reference electrode channel that iscreated using a flex circuit according to an embodiment of theinvention.

FIG. 2 is a top view of a flex circuit according to an embodiment of theinvention.

FIG. 3 is a top view of a mask that is used to cover the flex circuitshown in FIG. 2 according to an embodiment of the invention.

FIG. 4 is a top view showing a conductive material deposited into theopening of the mask according to an embodiment of the invention.

FIG. 5 is a top view of a mask that is used to cover a portion of theconductive material and the mask shown in FIG. 4 according to anembodiment of the invention.

FIG. 6 is a flow chart showing a method of creating the referenceelectrode channel of FIG. 1 according to an embodiment of the invention.

FIG. 7 is a cross-section view of a reference electrode channel that iscreated using a flex circuit according to an embodiment of theinvention.

FIG. 8 is a top view of a flex circuit according to an embodiment of theinvention.

FIG. 9 is a top view of a mask that is used to cover the flex circuitshown in FIG. 8 according to an embodiment of the invention.

FIG. 10 is a top view showing a conductive material deposited into theopening of the mask according to an embodiment of the invention.

FIG. 11 is a top view of a mask that is used to cover the conductivematerial and the mask shown in FIG. 10 according to an embodiment of theinvention.

FIG. 12 is a flow chart showing a method of creating the referenceelectrode channel of FIG. 7 according to an embodiment of the invention.

DETAILED DESCRIPTION

The invention is directed toward using a flex circuit to create areference electrode channel. The flex circuit has a reference electrodethat is masked and imaged onto a substrate. A first mask is deposited onthe substrate. The first mask may have an opening that has a first endthat exposes a portion of the reference electrode and a second end thatexposes a portion of the substrate. The opening forms a referenceelectrode channel. A conductive material may be deposited into theopening of the first mask. A second mask is deposited on the first maskand the conductive material. The second mask may have an opening thatexposes a portion of the conductive material that is over the substrate.

FIG. 1 is a cross-section view of a reference electrode channel that iscreated using a flex circuit according to an embodiment of theinvention. The flex circuit 100 may include a substrate 110, a trace120, and a reference electrode 125. The trace 120 and the referenceelectrode 125 may be masked and imaged onto the substrate 105. Forexample, the trace 120 and the reference electrode 125 may be formed onthe substrate 105 using screen printing or ink deposition techniques.The trace 120 and the reference electrode 125 may be made of a carbon,copper, gold, graphite, platinum, silver-silver chloride, rhodium, orpalladium material.

A first mask 130 may be applied or deposited over a portion of thesubstrate 110 and over the trace 120. The first mask 130 may have anopening 135 that expose a portion of the reference electrode 125 and aportion of the substrate 110. The opening 135 forms the referenceelectrode channel. A conductive material 140 is deposited in the opening135 to cover the exposed portion of the reference electrode 125 and theexposed portion of the substrate 110. A second mask 150 may be appliedor deposited over the first mask 130 and the conductive material 140.The second mask 150 may have an opening 160 over a portion of theconductive material 140 that is over the substrate 110. The opening 135is positioned along a first axis or plane and the opening 160 ispositioned along a second axis or plane. The first axis or plane is notcoincident with the second axis or plane. Hence, the first axis or planeis vertically and/or horizontally offset from the second axis or plane.

The opening 160 is the measurement site and allows a fluid of interest(e.g., blood, urine, etc.) to come into contact with the conductivematerial 140 to complete the measurement circuit with another measuringelectrode (not show) in contact with the same fluid. The conductivematerial 140 stabilizes the reference potential in several ways. Theconductive material 140 may provide known silver and chloride ionactivity, for example, (in the case of a silver-silver chloridereference design) to maintain a stable potential. The conductivematerial 140 should offer sufficient diffusion resistance to inhibitloss of desired ions to the fluid of interest, while simultaneouslyinhibiting migration of unwanted ions toward the active surface of thereference electrode 125. Spacing the opening 160 a sufficient distancefrom the reference electrode 125, as shown in FIG. 1, enhances thisdiffusion resistance. Finally, the conductive material 140 may provide apredictable junction potential at the interface with the fluid ofinterest which facilitates accurate electrochemical measurements usingthe reference electrode 125.

FIG. 2 is a top view of a flex circuit 100 according to an embodiment ofthe invention. The trace 120 and the reference electrode 125 may be madeof a conductive material such as a silver-silver chloride (Ag/AgCl)material and may be formed on the substrate 110 using photolithographyor printing techniques (610). For example, the trace 120 and thereference electrode 125 may be formed on the substrate 110 using screenprinting or ink deposition techniques. The substrate 110 may be aflexible dielectric substrate such as a polyimide. The trace 120 may beused to connect to a measurement device (not shown) such as apotentiostat. The trace 120 is used to measure a potential from thereference electrode 125 using the measurement device. Even though FIG. 1shows the flex circuit 100 having one trace 120 and one referenceelectrode 125, the flex circuit 100 may have more than one trace andmore than one electrode.

FIG. 3 is a top view of a mask 130 that is used to cover the flexcircuit 100 shown in FIG. 2 according to an embodiment of the invention.The mask 130 may be made of a dielectric material such as aphotoimagable epoxy or an ultraviolet curable epoxy material. The mask130 is deposited over the substrate 110 and has a rectangular opening135 that has a first end 135 a that exposes a portion of the referenceelectrode 125 and a second end 135 b that exposes a portion of thesubstrate 110 (620). The rectangular opening 135 may have a length ofbetween about 0.10-0.20 inches and a width of between about 0.010-0.020inches. The length-to-width ratio of the rectangular opening 135 may bein the range of between about 4:1 to 12:1. In one embodiment, the mask130 covers the entire top surface of the flex circuit 100 except for therectangular opening 135. The mask 130 may have a thickness of betweenabout 0.005 inches and about 0.02 inches. The first end 135 a of theopening 135 is positioned directly above the electrode 125 so that theelectrode 125 is exposed or visible through the opening 135 of the mask130. Lithography techniques may be used to deposit or place the mask 130on the flex circuit 100.

FIG. 4 is a top view showing a conductive material 140 deposited intothe opening 135 of the mask 130 according to an embodiment of theinvention. The conductive material 140 is deposited in the opening 135to cover and to come into direct contact with the exposed portion of thereference electrode 125 and the exposed portion of the substrate 110(630). The conductive material 140 may be a conductive fluid, aconductive solution, a conductive gel, a salt containing gel, aconductive polymer containing potassium chloride (KCl) with a smallamount of silver ion (Ag⁺), or a material having conductive properties.For the case of a silver-silver chloride reference electrode 125,addition of a trace of silver nitrate solution to a matrix containingpotassium chloride precipitates some amount of silver chloride withinthe conductive matrix, but maintains a silver ion concentration at aconstant amount according to the solubility product of silver chloride,which is 1.56×10⁻¹⁰.

FIG. 5 is a top view of a mask 150 that is used to cover a portion ofthe conductive material 140 and the mask 130 shown in FIG. 4 accordingto an embodiment of the invention. The mask 150 may be made of adielectric material such as a photoimagable epoxy or an ultravioletcurable epoxy material. The mask 150 has an opening 160 that exposes aportion of the conductive material 140 that forms a working surface toreceive a fluid of interest (640). Lithography techniques may be used todeposit or place the mask 150 on the mask 130 and the conductivematerial 140. FIG. 6 shows a flow chart of the method of creating thereference electrode channel corresponding to FIGS. 1-5 as describedabove.

FIG. 7 is a cross-section view of a reference electrode channel that iscreated using a flex circuit according to an embodiment of theinvention. The flex circuit 200 may include a substrate 210, traces 220and 230, a reference electrode 225, and a working electrode 235. Thetraces 220 and 230, the reference electrode 225, and the workingelectrode 235 may be masked and imaged onto the substrate 210. Forexample, the traces 220 and 230, the reference electrode 225, and theworking electrode 235 may be formed on the substrate 210 using screenprinting or ink deposition techniques. The traces 220 and 230, thereference electrode 225, and the working electrode 235 may be made of acarbon, copper, gold, graphite, platinum, silver-silver chloride,rhodium, or palladium material.

A first mask 240 may be applied or deposited over a portion of thesubstrate 210 and over the traces 220 and 230. The first mask 240 mayhave an opening 250 that expose a portion of the reference electrode225, a portion of the working electrode 235, and a portion of thesubstrate 210. The term “channel” (shown as channel 255) may be used torefer to the portion between the reference electrode 225 and the workingelectrode 235. Hence, the opening 250 may form the reference electrodechannel. A conductive material 260 is deposited in the opening 250 tocover and to come into direct contact with the exposed portion of thereference electrode 225 and up to the edge of the exposed portion of thesubstrate 210. A second mask 265 may be applied or deposited over thefirst mask 240 and the conductive material 260. The second mask 265 mayhave an opening 270 over a portion of the working electrode 235. Thereference electrode 225 is positioned along a first axis or plane andthe working electrode 235 is positioned along a second axis or plane.The first axis or plane is not coincident with the second axis or plane.Hence, the first axis or plane is vertically and/or horizontally offsetfrom the second axis or plane.

The opening 270 is the measurement site and allows a fluid of interest(e.g., blood, urine, etc.) to come into contact with the workingelectrode 235 and the conductive material 260 for a more accuratemeasurement. The conductive material 260 stabilizes the referencepotential in several ways. The conductive material 260 may provide knownsilver and chloride ion activity for example (in the case of asilver-silver chloride reference design) to maintain a stable potential.The conductive material 260 should offer sufficient diffusion resistanceto inhibit loss of desired ions to the solution, while simultaneouslyinhibiting migration of unwanted ions toward the active surface of thereference electrode 225. Spacing the opening 270 a sufficient distancefrom the reference electrode 225, as shown in FIG. 7, enhances thisdiffusion resistance. In addition, the opening 270 communicates directlywith the end of the conductive material 260 at a smaller opening 275.The proximity of the smaller opening 275 to the working electrode 235makes this embodiment ideal for situations where the solution resistancebetween the reference electrode and the working electrode needs to bekeep at a minimum, such as in the case of a 3-electrode amperometriccell, for example.

FIG. 8 is a top view of a flex circuit 200 according to an embodiment ofthe invention. The traces 220 and 230, the reference electrode 225 andthe working electrode 235 may be made of a conductive material such as acopper material, a platinum material, a silver-silver chloride (Ag/AgCl)material and are formed on the substrate 210 using masking andphotolithography techniques (1210). For example, the traces 220 and 230,the reference electrode 225, and the working electrode 235 may be formedon the substrate 210 using screen printing or ink deposition techniques.The substrate 210 may be a flexible dielectric substrate such as apolyimide. The traces 220 and 230 may be used to connect to ameasurement device (not shown) such as a potentiostat. The traces 220and 230 may be used to carry voltage or current from the referenceelectrode 225 and the working electrode 235 to the measurement device.

FIG. 9 is a top view of a mask 240 that is used to cover the flexcircuit 200 shown in FIG. 8 according to an embodiment of the invention.The mask 240 may be made of a dielectric material such as aphotoimagable epoxy or an ultraviolet curable epoxy material. The mask240 is deposited over the substrate 210 and has a rectangular opening250 that has a first end 250 a that exposes a portion of the referenceelectrode 225, a second end 250 b that exposes a portion of the workingelectrode 235, and a channel or an area 255 between the referenceelectrode 225 and the working electrode 235 that exposes a portion ofthe substrate 210 (1220). The rectangular opening 250 may have a lengthof between about 0.10-0.20 inches and a width of between about0.010-0.020 inches. The length-to-width ratio of the rectangular opening250 may be in the range of between about 4:1 to 12:1. In one embodiment,the mask 240 covers the entire top surface of the flex circuit 210except for the rectangular opening 250. The mask 240 may have athickness of between about 0.005 inches and about 0.02 inches. In oneembodiment, the first end 250 a of the opening 250 is positioneddirectly above the reference electrode 225 so that the referenceelectrode 225 is exposed or visible through the opening 250 of the mask240. In one embodiment, the second end 250 b of the opening 250 ispositioned directly above the working electrode 235 so that the workingelectrode 235 is exposed or visible through the opening 250 of the mask240. Lithography techniques may be used to deposit or place the mask 240on the flex circuit 200.

FIG. 10 is a top view showing a conductive material 260 deposited intothe opening 250 of the mask 240 according to an embodiment of theinvention. The conductive material 260 is deposited in the opening 250to cover and to come into direct contact with the exposed portion of thereference electrode 225 and in the area 255 between the referenceelectrode 225 and the working electrode 235 (i.e., on the exposedportion of the substrate 210) (1230). In one embodiment, a screenablegel or a conductive polymer is applied in the opening 250 to cover andto come into direct contact with the exposed portion of the referenceelectrode 225 and in the area 255 between the reference electrode 225and the working electrode 235. The conductive material 260 may be aconductive fluid, a conductive solution, a conductive gel, a saltcontaining gel, a conductive polymer containing potassium chloride (KCl)with a small amount of silver ion (Ag⁺), or a material having conductiveproperties. The conductive material 260 may form a salt channel or areference electrode channel.

FIG. 11 is a top view of a mask 265 that is used to cover the conductivematerial 260 and the mask 240 shown in FIG. 10 according to anembodiment of the invention. The mask 265 may be made of a dielectricmaterial such as a photoimagable epoxy or an ultraviolet curable epoxymaterial. The mask 265 has an opening 270 that exposes a portion of theworking electrode 235 and an edge of the conductive material 260, whichforms a space to receive a fluid of interest. Lithography techniques maybe used to deposit or place the mask 265 on the mask 240 and theconductive material 260 (1240). FIG. 12 shows a flow chart of the methodof creating the reference electrode channel corresponding to FIGS. 7-11as described above.

While certain exemplary embodiments have been described and shown in theaccompanying drawings, it is to be understood that such embodiments aremerely illustrative of and not restrictive on the broad invention, andthat this invention not be limited to the specific constructions andarrangements shown and described, since various other changes,combinations, omissions, modifications and substitutions, in addition tothose set forth in the above paragraphs, are possible. Those skilled inthe art will appreciate that various adaptations and modifications ofthe just described embodiments can be configured without departing fromthe scope and spirit of the invention. Therefore, it is to be understoodthat, within the scope of the appended claims, the invention may bepracticed other than as specifically described herein.

1. A sensor, comprising: a substrate; a reference electrode formed on afirst portion of the substrate, the reference electrode being made of afirst conductive material; a first mask located over the substrate, thefirst mask having an opening that exposes the reference electrode and asecond portion of the substrate; a second conductive material located inthe opening in the first mask, the second conductive material being indirect contact with the reference electrode; and a second mask locatedover the second conductive material, the second mask having an openingover the second portion of the substrate, the opening exposing a portionof the second conductive material, which forms a working surface toreceive a fluid of interest.
 2. The sensor of claim 1, wherein the firstconductive material is selected from a group consisting of a carbon,copper, gold, graphite, platinum, silver-silver chloride, rhodium, andpalladium material.
 3. The sensor of claim 1, wherein the secondconductive material is selected from a group consisting of a conductivefluid, a conductive solution, a conductive gel, a salt containing gel,and a conductive polymer containing potassium chloride with a smallamount of silver ion.
 4. A sensor, comprising: a substrate; a referenceelectrode formed on a first portion of the substrate and a workingelectrode formed on a second portion of the substrate, the referenceelectrode being made of a first conductive material; a first masklocated on the substrate, the first mask having an opening that exposesthe reference electrode, the working electrode, and an area between thereference electrode and the working electrode; a second conductivematerial located on the reference electrode and in the area between thereference electrode and the working electrode; and a second mask locatedon the second conductive material.
 5. The sensor of claim 4, wherein thereference electrode is made of a silver-silver chloride material and theworking electrode is made of a platinum material.
 6. The sensor of claim4, wherein the reference electrode is made of a material that isselected from a group consisting of a carbon, copper, gold, graphite,platinum, silver-silver chloride, rhodium, and palladium material. 7.The sensor of claim 4, wherein the working electrode is made of amaterial that is selected from a group consisting of a carbon, copper,gold, graphite, platinum, silver-silver chloride, rhodium, and palladiummaterial.
 8. The sensor of claim 4, wherein the second conductivematerial is selected from a group consisting of a conductive fluid, aconductive solution, a conductive gel, a salt containing gel, and aconductive polymer containing potassium chloride with a small amount ofsilver ion.
 9. A sensor, comprising: a substrate; a reference electrodeformed on a first portion of the substrate, the reference electrodebeing made of a first conductive material; a first mask located over thesubstrate, the first mask having an opening that exposes the referenceelectrode and a second portion of the substrate; a second conductivematerial located in the opening in the first mask, the second conductivematerial being in direct contact with the reference electrode; and asecond mask located over the second conductive material, the second maskhaving an opening over the second portion of the substrate, the openingexposing a portion of the second conductive material, which forms aworking surface to receive a fluid of interest wherein the opening inthe first mask is positioned along a first axis and the opening in thesecond mask is positioned along a second axis that is not coincidentwith the first axis.
 10. The sensor of claim 9, wherein the first axisis horizontally offset from the second axis.
 11. The sensor of claim 9,wherein the first axis is vertically offset from the second axis. 12.The sensor of claim 9, wherein the first conductive material is selectedfrom a group consisting of a carbon, copper, gold, graphite, platinum,silver-silver chloride, rhodium, and palladium material.
 13. The sensorof claim 9, wherein the second conductive material is selected from agroup consisting of a conductive fluid, a conductive solution, aconductive gel, a salt containing gel, and a conductive polymercontaining potassium chloride with a small amount of silver ion.
 14. Asensor, comprising: a substrate; a reference electrode formed on a firstportion of the substrate and a working electrode formed on a secondportion of the substrate, the reference electrode being made of a firstconductive material; a first mask located on the substrate, the firstmask having an opening that exposes the reference electrode, the workingelectrode, and an area between the reference electrode and the workingelectrode; a second conductive material located on the referenceelectrode and in the area between the reference electrode and theworking electrode; and a second mask located on the second conductivematerial, wherein the reference electrode is located along a first axisand the working electrode is located along a second axis that is notcoincident with the first axis.
 15. The sensor of claim 14, wherein thefirst axis is horizontally offset from the second axis.
 16. The sensorof claim 14, wherein the first axis is vertically offset from the secondaxis.
 17. The sensor of claim 4, wherein the reference electrode is madeof a silver-silver chloride material and the working electrode is madeof a platinum material.
 18. The sensor of claim 14, wherein thereference electrode is made of a material that is selected from a groupconsisting of a carbon, copper, gold, graphite, platinum, silver-silverchloride, rhodium, and palladium material.
 19. The sensor of claim 14,wherein the working electrode is made of a material that is selectedfrom a group consisting of a carbon, copper, gold, graphite, platinum,silver-silver chloride, rhodium, and palladium material.
 20. The sensorof claim 14, wherein the second conductive material is selected from agroup consisting of a conductive fluid, a conductive solution, aconductive gel, a salt containing gel, and a conductive polymercontaining potassium chloride with a small amount of silver ion.